A magnetic tunnel junction (MTJ) is a sandwich of two ferromagnetic metal layers separated by a thin insulating tunnel barrier. The tunneling resistance between the two ferromagnetic metal layers depends on the relative orientation of their magnetization directions (magnetic moments). The simplest form of a MTJ is one comprised of two ferromagnetic layers with different magnetic coercivities, which will be referred to as a "hard/soft" MTJ. One of the ferromagnetic layers, the fixed or "hard" layer, has a magnetic coercivity considerably greater than any magnetic field to which it will be subjected in use. The other ferromagnetic layer, the "soft" layer, has a lower coercivity such that its magnetic moment can be reversed by application of a small magnetic field in operation of the MTJ device. Thus, the MTJ device can operate as a memory storage element or cell wherein the two states of the memory cell correspond to the cases where the magnetic moments of the two ferromagnetic layers are aligned either parallel to one another or antiparallel to one another.
One problem with a hard/soft MTJ is that repeated reversal of the soft ferromagnetic layer's magnetization can demagnetize the hard ferromagnetic layer and thus erase the MTJ's memory. One way to form a stable MTJ device is to replace the hard ferromagnetic layer with an exchange-biased ferromagnetic layer, wherein one of the ferromagnetic layers is "pinned" by interface exchange coupling to an antiferromagnetic layer. Such a device will be referred to as an "exchange-biased" MTJ to distinguish it from a hard/soft MTJ. An exchange-biased MTJ is described in IBM's U.S. Pat. No. 5,650,958. The magnetization of the exchange-biased ferromagnetic layer in this exchange-biased MTJ is stable at least to 10.sup.7 cycles. However, the use of an exchange-biased ferromagnetic layer increases the complexity of the MTJ device and limits the temperature range over which the device can be subjected because of the relatively low blocking temperature of the antiferromagnetic layer, which is typically Mn-Fe. A hard/soft MTJ device, on the other hand, can operate over a much greater temperature range and is simpler to fabricate. In addition, a hard/soft MTJ device has an important advantage in that the magnetic moment of the hard ferromagnetic layer can be set in any arbitrary direction by merely applying a field sufficiently large to magnetize the hard ferromagnetic layer. This feature is described in IBM's pending application Ser. No. 08/757,175 filed Nov. 27, 1996.
Thus, what is needed is a hard/soft MTJ device that is able to withstand repeated magnetic cycling or reversals of its soft ferromagnetic layer.